The present invention relates to mass flowmeters and more particularly relates to a mass flowmeter that directly measures the difference between the mass flow rates of two fluid streams.
Flowmeters for measuring the mass flow rate of a fluid, such as a fuel, through the production of electrical signals representative of the mass flow rate are well known. Such flowmeters are shown in the U.S. Pat. Nos. 3,740,586 to Banks et al. and 3,807,229 to Chiles. Similarly, flowmeters for measuring the volumetric flow rate of a fluid through the production of electrical signals representative of such volumetric flow rate are also well known.
The fluid flowmeters with which the present invention is particularly concerned are those including a rotatable measurement assembly contained within a housing that is adapted for interconnection into the conduit through which the fluid is flowing.
In one type of volumetric flowmeter known to the prior art, the measurement assembly comprises a turbine having a plurality of turbine drive surfaces that are angled with respect to the direction of fluid flow through the housing so that the turbine is rotated by the fluid at a velocity proportional to the volumetric flow rate of the fluid.
In one type of mass flowmeter known to the prior art, the measurement assembly comprises a rotatable drum, an impeller for imparting angular momentum to the fluid, and a resilient torsion spring interconnecting the drum and the impeller. The impeller, because of the torsion spring coupling, will lag behind the first member by a deflection angle dependent upon the angular momentum imparted to the fluid. The deflection angle is therefore proportional to the mass rate of fluid flow through the impeller and measurement of the deflection angle will provide an output that is proportional to mass flow rate. The drum may be driven by a motor drive assembly, or may be driven by the fluid itself, through a turbine drivingly connected to the drum. The latter type of mass flowmeter, also known as a motorless mass flowmeter, is disclosed in U.S. Pat. No. 3,877,304 to Vetsch, issued on Apr. 15, 1975, and assigned to the assignee of the present invention. Typically, in mass flowmeters of the type described above, magnets are located on the periphery of both the drum and the impeller. The magnets induce pulse energy into a pair of stationary pick-off coils positioned adjacent the path of the magnets on the rotating drum and the deflected impeller. The signals induced into the coils will be out of phase by a factor proportional to the deflection angle between the drum and the impeller. The phase difference between successive pulses from the drum pick-off coil and the impeller pick-off coil is therefore proportional to the mass flow rate of the fluid.
The mass flow rate is, in many instances, the most important information concerning the flow rate of a fluid. The mass flow rate of fuel, for instance, is a desired measurement that is monitored in engine-testing procedures to provide an indication of engine condition and in actual engine use to provide information concerning the rate at which fuel is being used. Prior art mass flowmeters, however, are incapable of accurately measuring the net flow or flow consumed by a process wherein only a portion of the supply flow is consumed, the balance of the flow being returned to its source. An example of an environment in which such net flow measurement is important is in a diesel engine fuel system where a variable portion of the fuel supplied to the diesel engine is actually consumed by the engine. The variable remaining portion of the fuel supplied is used for injector lubrication and cooling and is returned to the source of supply to be resupplied to the engine. As a typical example, 20 percent of the fuel supplied to the engine may actually be consumed, with 80 percent of the supplied fuel returning to the source of supply for resupply.
It is theoretically possible to use a first prior art mass flowmeter to measure the mass flow rate of the supply flow to such a diesel engine and to use a second prior art mass flowmeter to measure the mass flow rate of the return flow. The difference between the measured supply mass flow rate and the measured return mass flow rate (the differential mass flow rate) would then be indicative of the rate at which fuel is being consumed by the engine. However, in actual practice it turns out that such a technique leads to significant errors in the measurement of differential mass flow rate. To give an example, let it be assumed that a typical prior art mass flowmeter has an accuracy of plus or minus two percent (.+-.0.02). Let it be further assumed that the actual supply mass flow rate (MF.sub.1 actual) is equal to P lbs/hr. and that the actual return mass flow rate (MF.sub.2 actual) is equal to 0.8P lbs/hr. Therefore, the actual differential mass flow rate (or, the fuel consumed by the engine) is 0.2P lbs/hr. Taking the worst case situation where the flowmeter measuring the supply mass flow rate has an error of plus two percent (+0.02) and the flowmeter measuring the return mass flow rate has an error of minus two percent (-0.02), the measured supply mass flow rate, MF.sub.1 measured' would be (1.02).times.(P lbs/hr.) or 1.02 lbs/hr. and the measured return mass flow rate, MF.sub.2 measured' would be (0.98).times.(0.8P) or 0.784P lbs/hr. The measured differential mass flow rate, then, would equal MF.sub.1 measured minus MF.sub.2 measured', or 0.236P lbs/hr. By subtracting the actual differential mass flow rate (0.2P lbs/hr.) from the measured differential mass flow rate (0.236 lbs/hr.) the measured error can be seen to be 0.036P lbs/hr., which is a measurement error of eighteen percent (18%). It is important to note that an accuracy within .+-.2% is regarded by the industry as being of superior quality. Yet, even with such superior quality, the final measurement can have an error of 18%.
It is therefore an object of the present invention to provide a mass flowmeter that can accurately measure the differential between the mass flow rates of two fluid streams.
It is another object of the present invention to provide a single flowmeter unit that provides signals representative of the individual mass flow rates of each of two or more fluid streams as well as the differential mass flow rate between pairs of any two of the fluid streams.